An aromatic polyimide of condensation type is known to have high heat resistance, high electric insulation, high mechanical strength and high chemical resistance. Therefore, the aromatic polyimide is widely employed for preparing, for instance, a protective coat for an electronic part, a shielding membrane, or an interlayer insullation membrane. The aromatic polyimide is also employable in combination with a metal powder such as a silver powder or a copper powder to give an electroconductive paste, in combination with carbon black to give a resistance paste, or in combination with a colored pigment to give a colored ink.
The aromatic polyimide is generally produced by reacting an aromatic tetracarboxylic dianhydride or its derivative with an aromatic diamine in a polar organic solvent to give an aromatic polyamide acid (or polyamic acid, which is a precursor of an aromatic polyimide), coating the polyamide acid on a substrate or an article, and then heating the coated polyamide acid to 180.degree.-450.degree. C., optionally in the presence of a chemical imidation catalyst.
The polyamide acid solution shows an extremely high viscosity when its concentration is high. Therefore, the polyamide acid solution is generally adjusted to have a polyamide acid concentration of less than approximately 30 weight %. The polyamide acid solution having such a low concentration is disadvantageous when a relatively thick polyimide layer or film is manufactured.
U.S. Pat. No. 3,511,790, and Journal of Applied Polymer Science, Vol. 16, pp. 905-915 (1972) disclose a polyimide precursor solution (varnish) comprising a substituted benzophenonetetracarboxylic acid diester and an aromatic diamine which are dissolved in a polar organic solvent. Japanese Patent Publication No. 58-46276 and Japanese Patent Provisional Publication No. 62-227978 disclose an electroconductive paste employing the above-mentioned polyamide acid solution.
Japanese Patent Provisional Publication No. 61-42573 and Japanese Patent Provisional Publication No. 61-83229 (Japanese Patent Publication H4-1785) disclose a polyimide precursor solution which comprises an aromatic tetracarboxylic acid ester and an aromatic diamine having four benzene nuclei in its molecular structure (and optionally a diaminosiloxane). The polyimide obtained using the polyimide precursor solution shows a low glass transition temperature and a low heat decomposition temperature, and therefore shows relatively poor heat resistance.